The invention relates generally to the field of liquid-crystal display devices used in electronic equipment and more specifically to refreshing video images on such displays.
Liquid-crystal displays (LCDs) are commonly used in a variety of electronic devices. The nematic crystals which comprise liquid-crystal displays have extremely low current requirements making liquid-crystal displays ideal for battery powered portable device applications.
Liquid-crystal display devices generally contain a nematic fluid having fairly large linear molecules which exhibit bipolar characteristics. The bipolar molecules of a liquid-crystal will align themselves in response to an externally applied electric field. In the absence of an externally applied electric field the bipolar molecules of the nematic fluid align themselves according to the inherent electric field generated from other surrounding bipolar molecules thereby representing the lowest collective energy state for that particular group of similar molecules.
The orientation of the molecules of the nematic fluid may be modulated by an externally applied electric field. The initial orientation of the nematic fluid molecules is controlled by directionally etching the interior surfaces of the glass plates containing the nematic fluid in the liquid-crystal display. This etch controls the direction of the initial low energy orientation of the nematic fluid molecules.
The orientation of the bipolar molecules subtractively affects the polarization of light passing through the liquid-crystal display. Modulation of the bipolar molecules allows for control of the light passing through the display. The degree of polarization of the light passing through the display may be controlled within limits by controlling the intensity of the externally applied electric field. A pixel is defined by the placement of transparent electrodes between the glass plates which contain the nematic fluid. Thus the bipolar molecules in the liquid-crystal display may be modulated in varying patterns and sequences to create the desired display image.
In order to control the voltage signals to the large number of pixels in the liquid-crystal display array it is necessary to multiplex the signal applied to the display electrodes. Specifically, the video signal is time-division multiplexed so that any given pixel is accessed for only that portion of time that it receives an applied voltage. So long as the multiplex rate is great enough, the human eye cannot detect that the bipolar molecules are polarized for a only a fraction of the time. Upon removal of the externally applied field an excited nematic fluid molecule will gradually revert to the low energy orientation defined by the directional etch. Greater such relaxation times such as exhibited by supertwisted nematic displays result in higher contrast ratios thereby allowing for higher multiplex rates.
As portable computer terminals and electronic equipment become more powerful, there is a trend for the video displays to become larger. When a larger liquid-crystal display is desired, more pixels are required in the array, and the display signal is multiplexed among a greater number of electrodes. Therefore each individual pixel is accessed for a smaller portion of the time. When a pixel is accessed for a smaller length of time, the contrast of the video image, the difference in intensity between the dark and light portions of the picture, and picture quality are reduced.
One solution to this problem is to drive the nematic fluid molecules with a higher voltage at higher multiplex rates in order to improve the video contrast ratio. Ideally the product of the voltage application time and the applied voltage remains constant as the multiplex rate increases. However, the resulting increased contrast is limited by the physical characteristics of the nematic fluid. If voltage applied to the nematic fluid is too large the nematic fluid will undergo dielectric breakdown causing the electrode material to plate across the fluid thereby damaging the display. Additionally, the response time for the molecules to reorient themselves has a maximum limit such that the molecules are unable to respond as rapidly as they are driven.
Increased power consumption of the voltage conversion electronics is a problem of using higher driving voltages because the efficiency of the voltage converters decreases as the magnitude of required voltages increases. The limit of the physical characteristics of the nematic fluid also presents problems when voltages are increased to compensate for increased multiplex rates. These problems are amplified as the density of pixels utilized in liquid-crystal displays is increased because the greater number of pixels inherently requires greater multiplex rates.
In most video applications, only a small portion of the display changes from one video refresh cycle to the next. Additionally, much of the information contained on several lines is identical or a subset of information contained on another line. Consideration of this redundancy of video information would allow the multiplex rate for each control cycle to be maximized for a given number of pixels in the liquid-crystal display, thereby maximizing the contrast ratio and picture quality for the desired video image.